This invention relates generally to devices for interconnecting and/or cooling electrical components and, more particularly, to a power bus and heatsink for electrically connecting and cooling electrical devices, along with related methods.
Presently, power switching devices such as the insulated Gate Bipolar Transistor (xe2x80x9cIGBTxe2x80x9d) are commercially packaged as both xe2x80x9cdiscretexe2x80x9d and xe2x80x9cmodularxe2x80x9d parts. Discrete parts, as typified by the popular TO-247 package, as sold by International Rectifier, Inc., have advantages of low packaging cost, compact size and low termination inductance. A typical manufacturing cost of the TO-247 package (less die and lead bonds) is about $0.12, while the typical termination (lead) inductance for this package is approximately 6 nH. Limitations of discrete packaged parts include a lack of electrical isolation and limited current capabilities. The maximum lead current capability for the TO-247 package is approximately 60 A.
Modular packaging has not become standardized to the degree that discrete parts have. An example of a popular modular package is the Powerex CMxe2x80x94DY package. One advantage of this type of packaging is the capability of packaging large total die areas so that high current ratings (more than 1000 A) can be achieved. Other advantages of the modular package include electrical isolation between the semiconductors and the heat-transfer surface and the capability of combining multiple semiconductor die so that several functions can be achieved within a single module.
Compared with discrete packaging, modular packaging has a number of disadvantages, including increased package cost and increased termination inductance. For modular devices, typical packaging costs are approximately equal to the bare Silicon die costs, whereas for the discrete packaged devices, the package cost is frequently less than 5% of the die cost. Accordingly, the manufacturing cost per VA for modular devices is nearly twice that of discrete devices. Furthermore, as die costs continue to fall more rapidly than packaging costs, this cost ratio between modular and discrete parts is expected to increase with time.
The termination inductance associated with modular packaging is also an increasing problem, as both die current ratings and die switching speeds are increasing with time. The net result is that for modular parts, voltage ratings must be reduced significantly below the die voltage ratingxe2x80x94often more than 20%. In contrast, the required voltage derating for discrete packaged parts is negligible. This, in turn, adds to the cost advantage for discrete partsxe2x80x94and particularly to the cost average over time.
While discrete packaged parts have the stated inherent economic advantage over their modular counterparts, this advantage is presently more than offset by the costs associated with heatsinking, mounting and terminating these parts. In particular, where multiple discrete parts must be paralleled, suitable means must be used to insure current balancing and uniform die temperatures in order to ensure viable operation. Accordingly, a situation exists where the manufacturing costs for complete power systems could be significantly reduced if a technically and economically viable means were at hand for simultaneously interconnecting, heatsinking and mechanically supporting discrete semiconductor devices.
FIGS. 1a-1c illustrate a prior art design for power processing that is based on the use of semiconductor modules 50. Semiconductor modules 50 are mounted in thermal contact with heatsink 51 which has fluid inlet 53 and fluid outlet 52; semiconductor modules 50 are electrically connected to capacitors 56 via circuit board 57; electrical input termination is provided by buses 54 and 55; and semiconductor modules 50 are controlled by terminals 57. Advantages of this design include a low impedance interconnection between capacitors 56 and semiconductor modules 50, and an efficient use of space. However, the semiconductor modules themselves cost approximately twice the cost of equivalently rated discrete semiconductor parts.
FIGS. 2a and 2b illustrate a prior art design for power processing that is based on the use of discrete semiconductor devices 10. Discrete semiconductor devices 10 are horizontally mounted in thermal contact with heatsink 51; and discrete semiconductor devices 10 are electrically connected to capacitors 56 (and other components that are not shown) via circuit board 11. The advantages of this design include the low cost associated with the discrete semiconductor devices 10, the low impedance interconnections between capacitors 56 and discrete semiconductor devices 10, and the design""s compatibility with commercially available heatsinks. However, this design is subject to high assembly costs, current limitations imposed by the circuit board foil resistance, high repair cost and inefficient use of space. The assembly cost is particularly high due to the fact that components are located on both sides of the circuit board, which makes automated soldering difficult or impossible. Included in the cost is the securing of each semiconductor device to the heatsink with individual hardware.
FIGS. 3a and 3b illustrate a prior art design for power processing that is based on the use of discrete semiconductor devices 10. Discrete semiconductor devices 10 are vertically mounted in thermal contact with heatsink 51; and they are electrically connected to capacitors 56 (and other components not shown) via circuit board 11. The advantages of this design include the low costs associated with discrete semiconductor devices 10, a low impedance interconnection between capacitors 56 and discrete semiconductor devices 10, and a moderately efficient use of space. The disadvantages of this design include a high assembly cost, the current limitations imposed by the circuit board foil resistance; and a high repair cost. The assembly costs are particularly high due to the fact that components are located on both sides of the circuit board, which makes automated soldering difficult or impossible.
Accordingly, there has existed a definite need for an energizing and cooling system, and related methods, for simultaneously interconnecting, heatsinking and mechanically supporting discrete semiconductor devices. The present invention satisfies these and other needs, and provides further related advantages.
The present invention provides an energizing and cooling system, a related method of cooling, and related methods of producing and installing such a system. It advantageously provides for devices, such as electrical components, to be efficiently and economically installed and used, with uniform power levels and uniform cooling.
In accordance with the present invention, a structure is defined which provides for the electrical interconnection, cooling and mechanical support of discrete semiconductor parts. Key elements of this structure include a conventional circuit board, a fluid-cooled heatsink which mounts on the component side of the circuit board, a spring clip which forces semiconductor devices installed in the circuit board into thermal contact with both front and rear surfaces of the heatsink, and electrically conductive buses which interconnect the circuit board with various components. Assembly of this structure may be fully automated using conventional fabrication means such as automated component insertion and wave soldering equipment.
The heatsink is typically an extruded aluminum tube having a rectangular outer cross-section and two liquid-filled interior chambers separated by a common wall. Interior surfaces of the heatsink may contain fins which protrude into the liquid to enhance heat transfer. At one end of the heatsink, the two chambers are made contiguous, thus establishing fluid counter-flow with respect to the common wall. The interior fluid is circulated by an external pump while heat contained in the fluid is transferred to ambient air by an external radiator and air blower.
An advantage of using two heatsink chambers with counter-flowing fluid (as compared with a single chamber arrangement) is that a more uniform thermal environment is provided for the components that are cooled by the heatsink. This is particularly important where a number of semiconductor components are connected in parallel, as uniform temperature is a requirement for both static and dynamic current balancing. A second advantage of the counter-flow arrangement is that the fluid inlet and fluid outlet may be combined into a single unit which saves packaging space and cost.
Two alternative embodiments are identified for the heatsink. In the first alternative embodiment, an aluminun extrusion is cut to length and, at one end, both the fins and the common wall are machined such that a contiguous region is achieved which allows fluid flow to connect from one chamber to the next in this region. A cap is sealed to this end to confine the cooling fluid to the interior chambers. As an alternate variation of a cap, a plug can be placed within the heatsink to accomplish approximately the same function. In the second alternative embodiment two cast members are bonded or welded together as a clam shell; and interior surfaces of each member contain pins which project into the fluid flow. This latter construction can achieve superior heat transfer compared to the first, but it has higher fabrication costs.
Individual heatsink members of either type may be integrated with a common two-chambered manifold to accommodate cooling for large numbers of components, while achieving desired length to width ratios for the completed equipment. Alternate two-chambered manifold designs can incorporate various flow paths, such as a counterflow design that provides for a manifold with a main inlet and outlet that are closely spaced, or such as a straight through design having a main inlet and main outlet at opposite ends. Individual heatsink members could also be integrated with separate inlet and outlet manifolds that are configured to conform to the heatsink inlet and outlet configuration. In alternate configurations, a manifold and a plurality of heatsinks could be cast as a single piece.
Termination for high currents is achieved by a unique sheet metal bus having fingers, which project outwardly from one edge of the sheet, pass through aligned holes within the circuit board and are soldered to electrically conducting surfaces on the circuit board. With the addition of appropriate slots and/or cuts, current distribution to the individual fingers may be controlled such that desired current ratios are achieved. In particular, such slots and cuts may be used to help maintain uniform current flow to a multiplicity of paralleled semiconductor devices which connect to the circuit board. The bus structure may be fabricated by low-cost stamping means.
A prototype three phase 100 kVA IGBT switch-mode amplifier has been built using methods of this patent. External dimensions of the amplifier are 10xe2x80x3xc3x9710xe2x80x3xc3x973.5xe2x80x3 (fluid circulating pump, radiator and radiator fan are external to the above amplifier). Measured thermal impedance between device junctions and the cooling fluid is 0.011 degrees Centigrade per Watt and measured thermal impedance between junction and ambient is 0.016 degrees Centigrade per Watt (the radiator cross section is 12xe2x80x3xc3x9712xe2x80x3 and the air flow rate is 1800 ft/min).
Using the methods of this invention, complete power systems, such as invertors, amplifiers, regulators and the like, may be assembled by fully automated means. Typical assembly steps can include:
1. Insertion of components in circuit board including heat dissipating and associated components such as capacitors, resistors, diodes, control components and connectors
2. Installation of the heatsink (in a vertical motion downward relative to the circuit board with the heatsink""s tongues aligning and engaging with circuit board slots)
3. Installation of one or more spring clips over the heat dissipating parts (this causes the heat dissipating parts to be clamped to the heatsink)
4. Insertion of power terminating sheet buses (in a vertical motion downward with respect to the circuit board, and with the bus fingers passing through holes in the circuit board)
5. Wave-soldering the xe2x80x9cfoil sidexe2x80x9d of the circuit board
6. Deflux cleaning of the circuit board
The above method is not restricted in order, other than to require the insertion of all components to be inserted prior to the steps of wave soldering and deflux cleaning.
An alternative to the above method entails the use of a caddy, i.e., a dummy heatsink, in place of the heatsink in the above method. The caddy preferably includes grooves conforming to the components, so as to hold the components in a preferred position during the wave-soldering. After the soldering is complete, the spring clip and the caddy are removed. The heatsink may then be moved into place, and the components are then clipped to the heatsink using the spring clip. If more than one heatsink is to be used in either of the above methods, the heatsinks are preferably attached to the manifold prior to installation on the circuit board to simplify assembly.
Other embodiments of this invention can be configured to affect switch-mode power conversion, such as rectification, inversion, frequency conversion, regulation, power factor correction or amplification. Likewise, other embodiments can be configured to affect linear processes, such as linear amplification or linear regulation.
Other features and advantages of the invention will become apparent from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.